Spacewalking astronauts completed almost all of the greasy repairs on a gummed-up joint at the international space station on Saturday, leaving just a few chores behind for another day.

As spacewalk No. 3 was getting under way 225 miles up, a new recycling system for converting urine into drinking water broke down again.

It was the third day in a row that the urine processor inexplicably shut down. The problem appeared to be related to how a centrifuge is mounted in the contraption. The centrifuge is on mounts meant to minimize vibrations. As a solution, flight controllers have asked station commander Michael Fincke to remove the isolation mounts.

"Fantastic! That is something we can do," Fincke told Mission Control when he was briefed on the matter.

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The problem could limit the amount of recycled water that is brought back to Earth for testing aboard space shuttle Endeavour next weekend.

The $154 million water recycling system, delivered a week ago by the space shuttle, is essential for allowing more astronauts to live on the space station next year.

Saturday's spacewalk by Heidemarie Stefanyshyn-Piper and Stephen Bowen was considered the most grueling of the mission and had been expected to last the longest, focusing entirely on the clogged solar wing-rotating joint. The joint stopped working properly more than a year ago, after it became jammed with metal grit from grinding parts, and cannot keep the solar wings on the right side of the space station pointed toward the sun.

The astronauts got started on the unprecedented clean and lube job — and bearing replacements — on Tuesday.

Mission Control wanted to keep Saturday's spacewalk close to the seven-hour mark and, six hours in, told the astronauts to wrap up what they were doing and start heading back in. The remaining chores — cleaning and greasing one final section of the joint and installing the one more bearing — will be squeezed into the fourth and final spacewalk of the mission Monday. That's when astronauts will grease up the good rotary joint on the left side of the orbiting complex.

"We really appreciate how hard you're all working," Mission Control radioed. "I know it's painful to call it quits like that, but we think it's the right thing to do."

Their spacewalk lasted just three minutes shy of seven hours, enough to make it the longest of the mission — barely.

"Welcome back aboard our beautiful space station," said skipper Mike Fincke.

Stefanyshyn-Piper — who lost a $100,000 tool kit during Tuesday's spacewalk — had to share grease guns again with Bowen. To make up for the grease gun shortage, they took out a caulking gun normally reserved for repairs to the shuttle's heat shield, but didn't need it.

They carefully guarded all their tethers so nothing would get loose.

"OK. Tether, tether, tether," Stefanyshyn-Piper counted before moving on to another task. "Three tethers, and they're all closed."

As for the broken urine-recycling system, flight controllers and astronauts alike were disappointed when it stopped working again Saturday. Mission Control radioed up the bad news just before the spacewalk began.

"I'm very sorry to hear that," astronaut Sandra Magnus replied.

The rest of the recycling equipment was working fine, aside from a minor glitch or two, and the two crews created more condensation in order to collect some drinking water samples from that. The astronauts turned off the air conditioner on the Russian side of the space station, cranked up the heat in the docked shuttle, and waited for condensation to form on equipment in the American compartments.

Even though there would be a smaller sample of urine-converted water than desired, at least there would be samples of processed condensation, said flight director Brian Smith.

NASA wants samples of the recycled water returned aboard Endeavour in order to conduct tests and ensure that it's safe to drink. The equipment is supposed to run for at least 90 days before anyone takes a sip.

The space agency cannot expand the size of the space station crew from three to six unless the water recycling system is working. NASA still hopes for that to happen by June.

Endeavour is supposed to leave the space station on Thanksgiving.